September 28, 2013

Three steps to heaven – and a few pitfalls along the way…

Click on any image to see a larger version

Over the past few weeks, I took an in-depth look at the various Metabones lens adapters for Nikon lenses, and at their remarkable Speed Booster model in particular. What started as a simple evaluation – do these lens adapters live up to their claims and to my expectations? – ended up with a comprehensive series of test shootings.

My experiments are limited to the combination of Nikon (and a few third-party) F-mount lenses and a Fujifilm X-mount camera. Note however that the Speed Booster optics tested here are identical to the Sony NEX version, as both cameras use APS-C sized sensors.

What did I learn from my test images?

To start, a number of positive conclusions that confirm the manufacturer’s claims:

1. Using a full-frame lens on a Fuji X (APS-C) camera, the Metabones Speed Booster nearly restores the “original” field-of-view and depth-of-field experience of working with a full-frame camera

The final overall crop factor is reduced from 1.5x to 1.07x (rounded), meaning a 50mm lens on a Speed Booster will deliver the field-of-view equivalent to a 53mm lens. The corresponding shallower depth-of-field and bokeh rendering are available again as well.

X-Pro1 + Speed Booster + AF-S 50mm f/1.4G set to f/1.4

2. The Metabones Speed Booster adds one full f-stop in light capturing performance

A 50mm f/1.4 lens becomes a 36mm f/1.0; an 85mm f/1.8 turns into a 60mm f/1.3, a 180mm f/2.8 acts like a 128mm f/2.0… Great news for low-light shooters!

A discussed later however, with some lenses the available f-stop range may end up slightly reduced.

X-Pro1 + Speed Booster + AF-D 20mm f/2.8 set to f/8

3. The Metabones Speed Booster delivers excellent image quality

A large selection of lenses likely to be combined with a compact system camera shows a similar to slightly better optical performance in the center of the frame. The corner performance turns out to be similar to slightly lower (especially at wider apertures), and varies more with the specific lens itself.

All things considered, I can only conclude and confirm that the Speed Booster does not significantly degrade the performance of the attached lens (as some forum and blog posts sometimes suggest). This holds true for sharpness, vignetting, bokeh rendering, color fringing…

The characteristics of the lens attached of course play a big role, and so does its condition when it’s an older, well-used copy. Furthermore, you need to compare apples to apples: it makes little sense to put side-by-side images shot using the full-frame area with others from a crop sensor using only the center area of the lens, or with pictures made on a full-frame camera with its built-in image correction features enabled.

X-Pro1 + Speed Booster + AF-D 180mm f/2.8ED set to f/4

There are also a few minor negative remarks, all related to the operation of the aperture control ring shared by the Speed Booster and the Nikon-G-to-Fuji-X adapters:

The Metabones website claims: “Industry's best aperture ring covers an 8-stop range with half-stop markings, clearly indicating actual number of f-stops. No more guesswork!”

In reality, you do get 15 clicking positions, but not 15 half f-stop settings as the wording may suggest. The aperture increments are not equally spaced across the available f-stop range of the lens: the step values are bigger at the open aperture end and decrease towards the smaller apertures.

You can use my “magic numbers” scale to perform the translation:

5. You may not be able to use the full aperture range originally offered by the lens

The aperture ring design limits the control span to about 6 f-stops only (starting from fully open). If your lens offers more, the smallest f-stop(s) will no longer be available.

In addition, with some lenses (or lens/adapter combinations?) the fully open iris position may not be reached, losing ⅓ to ½ stop at the wide open end. That takes back a little from the one f-stop gain, at least for who is looking to shoot with a minimal DOF. Note that this is often also the case with other adapters equipped with a different form of aperture control mechanism.

If – like myself – you mostly have F-mount lenses that do have an aperture ring, you might have preferred a Speed Booster version without aperture control on the adapter…

Both “negatives” will likely not have a major impact on real-life use, as the smallest aperture settings are generally avoided, in light of diffraction effects.

Are the Metabones lens adapters worth the investment?

The manufacturing quality of the Metabones adapters is simply best-in-class. Each adapter is precise, well-finished, robust, and a pleasure to operate. The Arca-compatible tripod mount is a nice feature, certainly with heavier or longer lenses.

Looking at the Speed Booster, the resulting images come very close to the “experience” obtained with a full-frame camera, and the image quality of the attached lens is largely preserved. The extra f-stop is a welcome addition to that. The Speed Booster helps to fill gaps in Fujifilm’s current XF lens line-up, at least until new lenses are added to the roadmap and become available (and the extra cash is spent).

In the end, the added value of a Speed Booster will vary from user to user. How extensive is your Fujifilm XF/XC lens collection, and how wide and fast will you let it grow in the future? How many Nikon F-compatible lenses do you already own, or do you plan to acquire some mint used copies to extend your lens line-up? Questions only you can answer for yourself…

The three Metabones lens adapters have found a warm place within my X-camera system. I use them on a regular basis with primes between 17mm and 180mm. The Speed Booster has opened up interesting opportunities, especially with fast lenses in the normal to short-telephoto focal range. Some of that may change when Fujifilm releases new XF lenses – the XF 56mm f/1.2 comes to mind immediately. But I guess all three adapters I acquired – and the Speed Booster in particular – will see plenty of action in the coming months.

Image quality reporting continued

Click on any image to see a larger version

TEST THREE: vignetting

Does a Speed Booster cause a visible increase in lens vignetting?

Most lenses show some noticeable level of vignetting at open and wide apertures. We all know that, and this effect was clearly present in the samples presented before. Furthermore, fast (f/1.4) lenses are known to suffer from vignetting more than others.

When (Nikon) lenses are mounted on lens adapters, there is no information about the working aperture passed on electronically to the camera. As a result, there is no in-camera image processing to compensate e.g. for vignetting, nor is any information included in the associated EXIF data to control compensation in post-processing software. In other words: the correction of vignetting, distortion, chromatic aberration and any other undesired effect is fully left with the photographer.

Today we all are used to cameras and workflows that silently take care of these matters – directly, or in post. And indeed, so do Fujifilm X-cameras when coupled with XF/XC lenses. So we easily forget that our DSLRs as well include some corrective image processing that we take for granted.

A Nikon camera like my D700 has a built-in vignette compensation that impacts the RAW (NEF) files. To make camera-to-camera comparisons a bit fairer, I have disabled this function on my D700 for these vignetting tests.

Images from various test series indicate that the level of vignetting with the same lens attached to an X-camera with Speed Booster or used directly on a DSLR (with vignetting control disabled) is very comparable. The composite image below shows the corner shading of the AF-S 50mm f/1.4G at various apertures on both camera configurations, and again illustrates the virtually identical vignetting effect:

We can conclude that – at least with this lens – the Speed Booster does not introduce any significant additional vignetting. That also means that whatever corner shading remains can easily be eliminated (if desired) through normal post-processing actions.

In the Speed Booster white paper and in follow-up discussions on forums, Brian Caldwell explains that the corner performance of the Speed Booster depends on the exit pupil distance of the lens. The exit pupil is the virtual image of the iris opening in an optical system as seen from whatever comes behind it. The exit pupil distance is not directly related to the focal length, and – unfortunately – is hardly ever documented within the lens specifications.

The optical design of the APS-C Speed Booster (shared by the Fuji X and Sony NEX mount versions) is optimized for lenses with an exit pupil distance in the 50-85mm range. This is perfect for most wide-angle, normal, and some short telephoto SLR lenses. Lenses with exit pupil distances beyond 100-110mm can be expected to show weaker corner performance. Here we can start seeing "hard" vignetting in the extreme corners, especially at small apertures. Longer telephoto lenses usually fall in that category, but there are also some examples at shorter focal lengths.

Only one lens in my test set showed a severe problem: the AF-S 70-200mm f/2.8G VR (first generation). The collection of test images below shows dark areas in the corners that do not disappear when closing the aperture, but rather become more sharply delimited. The effect is visible at all focal lengths across the zoom range, and does not seem to vary with the focus distance either.

It is possible that with the 70-200mm something is blocking part of the exit pupil image to properly hit the Speed Booster optics. The rearmost optical element of that lens indeed lies rather deep into the barrel. But no visible corner clipping occurs with the AF-D 180mm f/2.8ED, even though its rearmost element lies even deeper.

TEST FOUR: bokeh

One of the more attractive benefits of the Speed Booster is restoring the narrower depth-of-field (DOF) as experienced with full-frame lenses with full-frame sensor cameras, and with that the creation of a pleasing creamy bokeh in the out-of-focus areas.

What is the effect of the Speed Booster optics on a given lens’ bokeh? I have evaluated this using another simple but colorful test scene, with plenty of reflective materials.

Here are the results (and 100% crops) comparing the bokeh of a number of lenses when used directly on a D700 to the bokeh obtained with an X-Pro1/Speed Booster combo, starting with the AF-D 50mm f/1.4:

Next, looking at the AF-D 85mm f/1.8:

And finally the results for a longer telephoto lens, the AF-D 180mm f/2.8ED:

Conclusion: there is no significant change to the bokeh rendering of (at least these tested) lenses when used with a Speed Booster.

TEST FIVE: bokeh fringing

Bokeh fringing is caused by longitudinal chromatic aberrations (non-coinciding focal planes of the various colors) and is a common issue with relatively fast glass. The halos surrounding out-of-focus hard edges have different colors - magenta in front of the focus point and green beyond.

To learn more about this type of potential image degradation, I captured images from a SpyderLENSCAL autofocus alignment target for just a few lenses. The camera position was changed for each half image in the pair to have the same area reproduced in the crops. Only the center of the frame was examined. Note the shallower DOF in the Speed Booster images (left parts).

The results speak for themselves: no significant (center) quality loss from the Speed Booster.

TEST SIX: color reproduction

Every optical element we add to the system may alter the color reproduction characteristics of the overall system. I captured an X-Rite ColorChecker Passport target with and without the Speed Booster. White balance was taken from the left image, and the color temperature and tint values were transferred to the right (Speed Booster) image.

The Speed Booster introduces a very light orange cast. In Lightroom, it takes just a -50 temperature, -3 tint adjustment to optimally white balance the Speed Booster version. No obstacle in any post-processing workflow!

TEST SEVEN: infinity focus

It is very important that a lens adapter offers an absolutely correct (and uniform) flange focal distance. If the register distance is too long, it will no longer be possible to focus a lens at infinity.

The presence of optical elements in the Speed Booster potentially could further introduce problems.

I have not found any issue with any of my 20+ lenses checked.

(Note that it is important not to confuse matters here: infinity focus problems mean that you cannot anymore obtain a sharp image from very distant subjects. It does NOT mean that you can simply align the infinity symbol to the focus mark on your lens and count on having a sharp picture. Many lenses – and certainly older ones – may have their true infinity position before or even beyond the infinity mark.)

Even if an issue would be found, the Metabones web page describes a procedure to manually adjust infinity focus for a specific lens (risking however to make things worse for most if not all others…): the Speed Booster optics can be moved inside the housing after loosening a locking screw. Only for the brave of heart!

NOT TESTED: distortion

In a forum post, Brian Caldwell indicated that the APS-C Speed Booster adds about 1.5% of barrel distortion. That is not a lot, but if the attached lens already has barrel distortion things obviously gets worse (if the lens has pincushion distortion, the Speed Booster may actually correct it).

September 27, 2013

Playing like the big boys…

Click on any image to see a larger version

TEST TWO: image sharpness – the hard way

“Natural” scenes, like the one in our first test, do not always have very sharp edges or high-contrast areas. That is why professional testers use dedicated high-resolution test targets, such as the widespread ISO 12233 chart.

I do not have a full-size “original” for this target, but some wed searching led me to a 18688x1168 pixel file: good enough to make an A4-sized print on photo paper (with my modest HP Photosmart office printer). Obviously a target of this size, resolution and quality will not allow to capture a lens’ true performance “by the numbers”, but it might do a reasonable job when comparing images shot with the Speed Booster, with a regular F- or G-adapter, or with a D700.

The (too) small size of my ISO 12233 copy forced me to at very short subject distances, with a narrow DOF, with hard to control parallelism between sensor and target, and suffering from the not-so-flat flat field reproduction by many lenses. Most lenses (unless designed for repro or macro photography) furthermore will not reach their optimal performance at nearby shooting distances. So I limited myself to comparing two local areas, one in the center and one closer to the bottom left corner, as indicated on the target image. I also opted to shoot each lens at one f-stop down from fully open, to reduce some of the effects mentioned.

For this test as well the camera was repositioned when changing lens and adapter configurations to keep a more or less equal subject area covered.

Testing the Nikon AF-D 50mm f/1.4

Not too bad… we surprisingly see an improved performance with Speed Booster also in the corner.

Testing the Nikon AF-S 50mm f/1.4G

Same finding here: about equal in the center, improvement in the corner.

Testing the Nikon AF 50mm f/1.8

I also compared with the image quality when using this lens on a D700:

Testing the Nikon AF-D 85mm f/1.8

Testing the Tamron 90mm f/2.8 SP Di Macro

I was curious to try my favorite macro lens: I know it has excellent sharpness across the frame at close distances, but it also has a very flat field reproduction. And that shows in these samples:

Bring on the magnifying glass…

Click on any image to see a larger version

Time to share my findings on optical performance and image quality.

I told you before that I am not at all into pixel peeping, but rather want to find out where and how I can make best use of my lens collection. Even so, I slowly slid into the trap of chasing details and worrying about test outcomes that did not always match up well with other results… So here comes a load of test results and sample images, collected during a few weeks of image quality evaluation.

One more time: caveat emptor!

I am not equipped, qualified nor experienced for in-depth image quality testing. So take my attempts for what they are worth: a good swing at discovering whether the Metabones Speed Booster lives up to its manufacturer’s claims;

Lacking the tools and skills for real optical measurements, I focus on comparisons between different camera/lens setups under similar conditions. That means however that each of the alternatives compared may suffer from its own inaccuracies and so lead to conclusions that may be influenced by less-than-ideal test conditions;

Everything is based on sample-of-one testing: one camera, one lens adapter, each lens… only.

So I urge you to look at my test results NOT as an absolute measurement of the Metabones Speed Booster’s capabilities, but rather as an indication of what it CERTAINLYCAN achieve!

I include some information on the test setups and on the processing of the results, so that you can make your own judgment on the outcomes as presented.

F-mount lenses were mounted on the X-cameras using either the Speed Booster, or via a Metabones G- or F-adapter (depending on lens type);

Aperture setting was done on the lens if an aperture ring was available; with G-lenses I set the aperture via the adapter ring following my “magic number” scale described earlier;

Vignette Control set OFF on the D700 (it does affect NEF files!);

When a 12Mpix D700 image is compared to a 16Mpix X-Pro1 image, the latter image is scaled down to the smaller pixel dimensions before extracting 100% crops, to keep the represented image areas alike;

All images were shot in RAW using AdobeRGB space and a fixed custom white balance setting;

Post-processing in Adobe Lightroom 5.2RC using 2012 (Current) process, Adobe Standard profile, all settings on default, no image adjustments other than specifically noted with each test.

TEST ONE: image sharpness

I assembled a small colorful scene in my familiar light tent. The center resp. (lower right) corner focus areas are indicated by the white center mark symbols. The grey card in the background was used to “equalize” the individual shots with respect to exposure and white balance (the D700 images have a different color rendering, due to in-camera processing or post-processing?).

A series of lenses was tested across a range of apertures (typically fully open through f/5.6). The camera was repositioned between shooting with and without the Speed Booster, to obtain a comparable reproduction size (no excessive effort was applied beyond being reasonable...).

The size of the scene was relatively small and the shooting distance (with 50mm lens on Speed Booster) around 50cm only. Shallow DOF, imperfect alignment, field curvature etc. all make this test rather difficult to execute well, and thus more subject to human error.

Because of the shallow DOF and the possible field curvature, the lenses were focused separately for the center and for the corner targets. Focus was left untouched when changing aperture. In some cases, multiple series were captured to select the images with the best focus.

Metabones and Brian Caldwell claim equal to better image sharpness in the center, and a slight degradation in the corners. Roger Cicala’s measurements largely confirm this. Both sources remark that the results may vary with individual lenses. Let’s find out for ourselves.

Testing the Nikon AF-D 50mm f/1.4

It might be more relevant to compare using 100% crops:

(The different size of the 100% crop area is a result from the downscaling of the 16Mpix X-Pro1 image, to end up with the same pixel dimensions as the crop from the 12Mpix D700 image)

The result fully open is not worse with the Speed Booster compared to without, and seems slightly worse compared with the D700 shot (manual focus error?).

Nikon’s 50mm f/1.4 lenses are not exactly known for their corner sharpness wide open; we see the predicted minor degradation when looking at the corner crops, though the result is certainly not bad at all! (remember that a separate series of images was shot, with adjusted focus, for center and for corner sharpness )

The relative differences still hold at f/5.6:

Note how using the Speed Booster – as expected! – reduces the DOF.

Testing the Nikon AF-S 50mm f/1.4G

This more recent 50mm lens is generally said to be slightly better than the AF-D 50mm f/1.4 in both center and corners, but still far from perfect. This lens reaches its maximum performance only around f/5.6.

For some reason I kept having problems to achieve acceptable focus in the corners, and I did not bother to completely redo the test for that. So center crop comparisons only…

Similar observations as before, also at f/5.6:

Slight advantage to the Speed Booster image for center sharpness.

Testing theAF 50mm f/1.8

A Nikon classic, and usually a solid performer.

Similar behavior in the center, slight degradation in the corner, almost gone by f/5.6:

Testing the Nikkor H·C 50mm f/2.0

How will this pre-AI fully manual lens from the early 70s hold its ground with the Speed Booster?

Again the same conclusion: slight improvement of the center sharpness with the Speed Booster.

Testing the Nikon AF-D 35mm f/2.0

Another classic, in the ‘conventional’ wide angle range.

Testing the Nikon AF-D 85mm f/1.8

One of my favorite AF-D lenses, currently filling a hole in the Fujifilm XF lens line-up.

The 85/1.8D shows itself as a strong performer fully open, taking good advantage of the Speed Booster in the center, and not losing much in the corner.